In the recent years of the advanced information society, communication apparatuses including mobile phones or wireless local area networks (LANs) that ensure the linearity of a transmission signal in a wide range of amplification of power, and operate with low power consumption, at the same time, are desired. For conventional communication apparatuses, a transmission circuit that outputs an accurate transmission signal and operates efficiently in all bandwidth, is used. However, for example, in a transmission circuit that generates a transmission signal using the modulation scheme including quadrature modulation (hereinafter referred to as “quadrature modulation circuit”), the linearity of a transmission signal and the efficiency of a transmission circuit have a relationship of trade off, and it is difficult to achieve the linearity of a transmission signal and the efficiency of a transmission circuit at the same time.
As a conventional transmission circuit that operates accurately and highly efficiently, for example, Patent Literature 1 discloses a transmission circuit using the envelope tracking method. FIG. 1 is a block diagram showing a configuration of conventional transmission circuit 10 disclosed in Patent Literature 1.
A transmission signal is input to power amplifier 15 via a quadrature modulator including delay circuit 16, digital to analog converter (DAC) 17, and up-converter 18. Further, envelope extraction section 11 extracts and outputs an envelope of the input transmission signal, as an envelope signal. The envelope signal receives bandwidth limitation in bandwidth limitation process section 12, and then is supplied to variable power supply section 14 via DAC 13. Variable power supply section 14 inputs the voltage based on the supplied signal to power amplifier 15. Power amplifier 15 amplifies the signal input from up-converter 18 to obtain an output signal.
Here, bandwidth limitation process section 12 limits the band so that the input envelope signal and the peak value of the output signal match. By making the envelope signal match the peak value of the output signal, it is possible to prevent power supply voltage (drain voltage or collector voltage) supplied to the power amplifier from variable power supply section 14 from lowering due to bandwidth limitation, avoiding deterioration of the linearity. Transmission circuit 10 lowers the required response speed of variable power supply section 14 using the envelope tracking method, without causing deterioration of the linearity of the amplifier.